The ultrasonic echolocation calls of bats are recorded by a variety of bat detector devices using full spectrum analysis and/or zero-crossing detection technology. Full spectrum devices are conventional digital audio recorders that record using high sample rates, e.g. typically between 192 kilohertz (kHz) and 500 kHz to resolve ultrasonic signals typically between 96 kHz and 250 kHz. Zero-crossing devices filter out unwanted low frequency signal content and then record the timings between a constant number of successive zero crossings from which the dominant frequency of narrowband ultrasonic signals as they change through time can be derived.
Rather than record continuously, bat detector devices typically wait for a triggering event such as the detection of an ultrasonic signal, and then record until either the signal is no longer detected or for a fixed maximum duration, typically up to several seconds. Triggering can be performed in either the analog or digital domains. Some devices analyze each triggered signal to determine if the signal is likely to contain the echolocation calls of bats or might be falsely triggered due to ultrasonic energy in rain, wind, and other conditions. If a signal is unlikely to contain echolocation calls, the device may opt to not save the recording, or delete the recording after it is saved, in order to save space and make post-processing more efficient. The process of selectively not saving or deleting unwanted recordings is called “scrubbing.”